1. Field of the Invention
The invention relates to a method for fabricating a vertical comb drive structure, more particularly to a method for fabricating a self-aligned vertical comb drive structure and the self-aligned vertical comb drive structure that is fabricated using the method.
2. Description of the Related Art
A micro scanning mirror is currently one of the most widely utilized devices in a micro-opto-electromechnical system (MOEMS). Typically, the micro scanning mirror is actuated by a vertical comb drive actuator. The vertical comb drive actuator includes two comb structures that are disposed respectively on different device layers. The comb structures include a plurality of comb finger portions that are inter-digitated. In operation, when a potential difference is applied to the comb structures, one of the comb finger portions (e.g., a movable comb finger portion) moves with respect to the other one of the comb finger portions (e.g., a fixed comb finger portion).
It is essential in the fabrication process that the two comb structures be accurately positioned to ensure a distance therebetween is sufficient for movement during operation. This is essential since, when a higher potential difference is employed, the movement of the movable comb finger portion may become unstable (e.g., sideways movement may occur). Additionally, the vertical comb drive actuator is typically scaled down to an extent where a distance between the movable comb finger portion and the fixed comb finger portion may be as small as a few micrometers. In such cases, misalignment of the comb structures may result in greatly reduced performance of the vertical comb drive actuator.
Conventionally, a vertical comb drive actuator is fabricated first using a photolithography process to define respectively patterns of the two comb structures on two opposite sides of a wafer. The two comb structures are then formed via etching the two opposite sides of the wafer. In this way, it may be difficult to accurately form the patterns on the two opposite sides of the wafer to accurately correspond to each other, and in turn the comb structures thus formed may be somewhat misaligned.
In order to address the aforementioned alignment issue, methods for fabricating a self-aligned vertical comb drive actuator have been proposed. For example, U.S. Pat. No. 6,612,029 and US Patent Application Publication No. 2007/0241076 disclose such a method. Conventionally, in such a method, the photolithography process is utilized to form two etch stop layers on one side of the wafer, each of the etch stop layers having a distinct etch selectivity. Afterward, the one side is etched to form two comb structures using the two etch stop layers, respectively.
However, the methods employed in those prior art references have a number of drawbacks. For example, with an increased demand for performance of the vertical comb drive actuator, an aspect ratio (the ratio of a depth to a width of the formed trench between adjacent fingers) is typically increased as well. Furthermore, since two comb structures are formed from one side of the wafer, an etch depth may be too deep such that lateral etching may occur, resulting in shapes of an inner comb structure being different from the expected shapes.
It is also noted that, in the second embodiment of US Patent Application Publication No. 2007/0241076, an etch stop layer is disposed between two device layers. Removing the etch stop layer may prove difficult due to the increased aspect ratio that may prevent the etching agent from reaching the etch stop layer. While isotropic etching may be used to address this issue, occurrence of lateral etching still results in shapes of the inner comb structure being different from the expected shapes.